Buggy code:
public abstract class DefaultCompilerAdapter implements CompilerAdapter { private static final FileUtils FILE_UTILS = FileUtils . getFileUtils ( ) ; protected Path src ; protected File destDir ; protected String encoding ; protected boolean debug = false ; protected boolean optimize = false ; protected boolean deprecation = false ; protected boolean depend = false ; protected boolean verbose = false ; protected String target ; protected Path bootclasspath ; protected Path extdirs ; protected Path compileClasspath ; protected Path compileSourcepath ; protected Project project ; protected Location location ; protected boolean includeAntRuntime ; protected boolean includeJavaRuntime ; protected String memoryInitialSize ; protected String memoryMaximumSize ; protected File [ ] compileList ; protected static final String lSep = System . getProperty ( "line.separator" ) ; protected Javac attributes ; public void setJavac ( Javac attributes ) { } public Javac getJavac ( ) { } protected Project getProject ( ) { } protected Path getCompileClasspath ( ) { } protected Commandline setupJavacCommandlineSwitches ( Commandline cmd ) { } protected Commandline setupJavacCommandlineSwitches ( Commandline cmd , boolean useDebugLevel ) { } protected Commandline setupModernJavacCommandlineSwitches ( Commandline cmd ) { } protected Commandline setupModernJavacCommand ( ) { } protected Commandline setupJavacCommand ( ) { } protected Commandline setupJavacCommand ( boolean debugLevelCheck ) { } protected void logAndAddFilesToCompile ( Commandline cmd ) { } protected int executeExternalCompile ( String [ ] args , int firstFileName ) { } protected int executeExternalCompile ( String [ ] args , int firstFileName , boolean quoteFiles ) { String [ ] commandArray = null ; File tmpFile = null ; try { if ( ( ( Commandline . toString ( args ) . length ( ) ) > 4096 ) && ( firstFileName >= 0 ) ) { PrintWriter out = null ; try { tmpFile = DefaultCompilerAdapter . FILE_UTILS . createTempFile ( "files" , "" , getJavac ( ) . getTempdir ( ) ) ; tmpFile . deleteOnExit ( ) ; out = new PrintWriter ( new FileWriter ( tmpFile ) ) ; for ( int i = firstFileName ; i < ( args . length ) ; i ++ ) { if ( quoteFiles && ( ( args [ i ] . indexOf ( "<seq2seq4repair_space>" ) ) > ( - 1 ) ) ) { <START_BUG> args [ i ] = args [ i ] . replace ( '\\' , '//' ) ; <END_BUG> out . println ( ( ( "\"" + ( args [ i ] ) ) + "\"" ) ) ; } else { out . println ( args [ i ] ) ; } } out . flush ( ) ; commandArray = new String [ firstFileName + 1 ] ; System . arraycopy ( args , 0 , commandArray , 0 , firstFileName ) ; commandArray [ firstFileName ] = "@" + tmpFile ; } catch ( IOException e ) { throw new org . apache . tools . ant . BuildException ( "Error<seq2seq4repair_space>creating<seq2seq4repair_space>temporary<seq2seq4repair_space>file" , e , location ) ; } finally { FileUtils . close ( out ) ; } } else { commandArray = args ; } try { Execute exe = new Execute ( new org . apache . tools . ant . taskdefs . LogStreamHandler ( attributes , Project . MSG_INFO , Project . MSG_WARN ) ) ; if ( Os . isFamily ( "openvms" ) ) { exe . setVMLauncher ( true ) ; } exe . setAntRun ( project ) ; exe . setWorkingDirectory ( project . getBaseDir ( ) ) ; exe . setCommandline ( commandArray ) ; exe . execute ( ) ; return exe . getExitValue ( ) ; } catch ( IOException e ) { throw new org . apache . tools . ant . BuildException ( ( ( "Error<seq2seq4repair_space>running<seq2seq4repair_space>" + ( args [ 0 ] ) ) + "<seq2seq4repair_space>compiler" ) , e , location ) ; } } finally { if ( tmpFile != null ) { tmpFile . delete ( ) ; } } } protected void addExtdirsToClasspath ( Path classpath ) { } protected void addCurrentCompilerArgs ( Commandline cmd ) { } protected boolean assumeJava11 ( ) { } protected boolean assumeJava12 ( ) { } protected boolean assumeJava13 ( ) { } protected boolean assumeJava14 ( ) { } protected boolean assumeJava15 ( ) { } protected Path getBootClassPath ( ) { } protected String getNoDebugArgument ( ) { } }

Fix:
args [ i ] = args [ i ] . replace ( File . separatorChar , '//' ) ;